WASHINGTON (CNN) -- Newsweek magazine backed away Sunday from a report that U.S. interrogators desecrated copies of the Quran while questioning prisoners at the Guantanamo Bay naval base -- an account blamed for sparking violent riots in Afghanistan and elsewhere.

At least 15 people were killed and dozens injured last week when thousands of demonstrators marched in Afghanistan and other parts of the Muslim world, officials and eyewitnesses said.

"We regret that we got any part of our story wrong, and extend our sympathies to victims of the violence and to the U.S. soldiers caught in its midst," Newsweek Editor Mark Whitaker wrote in the magazine's May 23 issue, out Sunday.

In an article assessing its coverage, the magazine wrote, "How did Newsweek get its facts wrong? And how did the story feed into serious international unrest?"

The Pentagon said last week it was unable to corroborate any case in which interrogators at the U.S. Naval Station in Guantanamo Bay, Cuba, defiled the Muslim holy book, as Newsweek reported in its May 9 issue.

"Top administration officials have promised to continue looking into the charges, and so will we," Whitaker said.

Pentagon spokesman Larry DiRita blamed Newsweek's report for the violent protests that broke out in Jalalabad, Afghanistan, Pakistan and other Muslim countries.

"People are dying. They are burning American flags. Our forces are in danger," he told CNN.

Newsweek said anger over the story spread after it was cited at a May 6 press conference in Islamabad, Pakistan, by Imran Khan, a Pakistani cricket legend and a critic of Pakistan President Pervez Musharraf.

In the story, the magazine cited sources as saying investigators looking into abuses at the military prison found interrogators "had placed Qurans on toilets, and in at least one case flushed a holy book down the toilet."

"Desecrating the Quran is a death-penalty offense" in Afghanistan and neighboring Pakistan, said Peter Bergen, a CNN terrorism analyst.

"There is clearly a lot of anti-American sentiment in Pakistan, less so in Afghanistan, but I think that this will feed into it," Bergen said.

Newsweek's Washington bureau chief, Dan Klaidman, said the apparent error was "terribly unfortunate," and he offered the magazine's sympathies to the victims.

But he said "different forces" were at work that helped spark the riots.

"It's clear that people seized on the Newsweek report to advance their own agendas, and that that was part of it," he said.

"But I also think that there's an enormous amount of pent-up and not-so-pent-up anti-American rage and sentiment in that region."

"There are a lot of people who think that our war on terror and our war in Iraq is a much wider war against Islam," he said.

At a Pentagon press conference Thursday, Gen. Richard Myers, chairman of the Joint Chiefs of Staff, cited U.S. commanders as saying the protests in Jalalabad, at least, were more about local politics than anti-American sentiment stirred by the Newsweek report.

The story's origins

Newsweek said Michael Isikoff, who reported the item with John Barry, became interested in the story after FBI e-mails that revealed an uglier side of life in Guantanamo were released late last year.

"Isikoff knew that military investigators at Southern Command [which runs the Guantanamo prison] were looking into the allegations," the article said.

"So he called a longtime reliable source, a senior U.S. government official who was knowledgeable about the matter.

"The source told Isikoff that the [investigators'] report would include new details that were not in the FBI e-mails, including mention of flushing the Quran down a toilet."

Whitaker wrote that before publishing the account the magazine approached two Pentagon officials for comment. One declined and the other challenged a different aspect of the report, Whitaker wrote.

Myers said at the Pentagon briefing Thursday the military was looking into the allegations.

He said investigators had so far been unable to confirm a "toilet incident, except for one case, a log entry, which they still have to confirm, where a detainee was reported by a guard to be ripping pages out of a Quran and putting [them] in the toilet to stop it up as a protest. But not where the U.S. did it."

On Friday, Newsweek said, DiRita phoned the magazine and said that investigators found no incidents involving Quran desecration.

A day later, Isikoff reached his source again, who said that although he remembered reading investigative reports about desecration of the Quran, including a toilet incident, "he could no longer be sure that these concerns had surfaced in the [Southern Command] report."

DiRita "exploded" when Newsweek informed him that one of the original sources behind the report had partially backed off the story, the magazine said.

"People are dead because of what this son of a bitch said," DiRita told Newsweek, according to the magazine's report. "How could he be credible now?"

DiRita confirmed the quote to CNN.

He said investigators have found nothing to support allegations that U.S. troops had desecrated copies of the Quran, but turned up one case he said has now led to stricter procedures at the prison camp.

In that case, a Quran fell to the floor during a routine search, he said. The book was encased in surgical mask, which prisoners at the facility are given to protect the book.

Camp commanders have since established stronger procedures when searching near a Quran, DiRita said -- including a rule that allows only Muslim troops, interrogators or chaplains to touch a copy.

But Newsweek said Isikoff has uncovered more allegations of Quran desecration.

One, from an attorney representing some of the detainees, provided some declassified notes indicating 23 detainees had tried to commit suicide in August 2003 when a guard dropped a Quran and stomped on it. (Full story)

Isikoff found two other references to Qurans being tossed into toilets or latrines, the magazine reported.

U.S. military officials said such claims are standard terrorist tactics.

"If you read the al Qaeda training manual, they are trained to make allegations against the infidels," Army Col. Brad Blackner told Newsweek.